help needed with shading applied to a certain part of a mesh?
You can:
Select faces of your mesh in edit mode and assign them different materials. This is the easiest, most straightforward way.
Or
Use texture paint to create a black/white mask for the two textures. Combine the two shaders (Put them in the same material) using a Mix Shader, and plug in this mask as the mix factor. This allows for more control, like smooth texture blending and shapes that are not limited to the object's geometry. However, this takes more work to do.